Trivial (?) question about molds

[nemo]

Hi,
I'm new in moldmaking, and I was wondering how a boat like this:
http://www.cherubiniyachts.com/cc20-...o-top-side.jpg
(Cherubini22 by E.Sponberg) can be built with the classic two molds, hull and deck: I'm concerned about the stern sides shape, which is convex. It maybe a trivial question, but I'm not sure if I can figure out how the hull can be built with a single mold, either male or female. And how is it possible to build an inboard flange for hull-deck joint? does it have to be laminated to the hull later?

[Herman]

For this shape it would be questionable: It could be lifted at the front, then slid forwoard slightly, to come out of the mould. However, a shape like this is likely to come out of a split mould. The splitline would be the keel/stern line. Be splitting the moulds, one can make much more complicated shapes than with a single mould.

About flanges to the inside: 2 options generally exist:
-split moulds, like discussed above.
-have a wide flange on the mould, facing outward. On this flange, a contraplate is bolted, potruding inward. Against this inbound flange the product flange is laminated.

[nemo]

Thanks, that was what I was thinking about too..
I've seen a sailboat built with a split mold...does it lead to an equal grade of safety of the structure, compared with a single mold, or it can be tricky in the junction? another flange needed I guess...

[Ssor]

My Islander sailboat hull was built in two molds I believe because when I was refitting her the joint was filled with fairing compound on the outside and heavily glassed on the inside. But with the deep full keel I don't think it would be possible to hand lay fiberglass into that shape in a one piece mold, it also has an inturned flange for the deck joint.

[Herman]

Mostly the boat is laminated with the mold halves together, so the boat is a single piece of fiberglass. Then the mold halves are unbolted and pulled of the hull one by one.

[Eric W. Sponberg]

Nemo,

The Cherubini Classic 20 is built in a 3-part hull mold--port and starboard halves, and a transom. The deck is one separate piece.

On the hull, all the mold parts are joined together at the beginning. When the mold parts are separated and the hull removed from the mold, there is a bit of flashing of gelcoat at all the mold joints. These are covered up with hardware in the finished boat so that you cannot see any seams--we did it that way on purpose.

Eric

[nemo]

Thanks for your answers, now it's more clear..
Now I would like to know how could I make this deck-hull connection in the stern area: the transom should be part of the hull molds or , since the curvature continuity between transom and deck, part of the deck mold? And the deck-hull joint should be done with a internal flange?

[Eric W. Sponberg]

Nemo,

I am not sure I understand your picture--not enough of the boat is showing. However, when designing mold, and the parts coming out of the mold, you have to plan for the joints, obviously, and figure out some way to hide those joints with other fittings or features, or otherwise incorporate those joints into the design. And, for an inward turning flange on the hull or deck, you have to plan for removable flanges on the mold so that the hull or deck can be removed.

For example, lets say that the hull is normally built with an inward turning flange about 4" wide (100 mm). The flange is rabbetted downward a bit for the inner 3" so that when the deck lands on the flange, the top surface of the deck and the top surface of the remaining 1" outboard of the hull flange are flush. When joined, an aluminum toe rail will be positioned over the joint between the hull and deck to hide it.

To make the hull, the hull mold has to have a removable flange mold. The hull mold itself will come up to, say, 3" (75 mm) below the corner at the sheer line, turning outward into a 4" (100 mm) wide outward turning flange. Another section of mold is made to mold the inward turning flange of the hull. It starts with an outward flange that mates with the hull mold flange, turns up the last 3" of the hull, goes over the sheer, and then inward to form the shape of the inward turning hull flange.

The hull is laid up with the flange molds in place. To remove the hull, the flange molds are unbolted from the hull mold, exposing the inward turning flange and the top 3" of the hull. The hull is removed, and there is a small flashing of gelcoat (or resin) sticking out right at the mold line. This can be addressed in two ways: 1) you carefully grind off the flashing, sand and polish the line all around the boat, or 2) you grind off the flashing and cover it with a decorative decal, something like an accent stipe of a different color that has been worked into the design.

So in your case, you have to figure out where you want the joint line between the hull and deck, and how you are going to treat it on the finished boat. You can make it a part of the design or you can hide it in some way. Remember that you don't have to restrict yourself on the number of mold parts, you can have as many as you want. The more you have, of course, the more expensive the mold is, but somewhere there is a happy medium on cost and ease of use of the mold. This is also why in fiberglass boat building, you have to be aware of how the parts are going to be made as well as how they look when finished. You have to keep joint lines and mold parts in mind when going through the design. This you are doing right now, and don't be surprised if you have to make some compromises on the design itself to make it easy to build.

I hope that helps.

Eric